The invention relates to a calcined catalyst for converting paraffin hydrocarbons into corresponding olefins by dehydrogenation, wherein the catalyst contains an oxidic, thermally stabilized substrate material and a catalytically active component that is applied to the substrate material. The invention also relates to a method for converting paraffin hydrocarbons into corresponding olefins, in which a stream of the paraffin hydrocarbons is mixed with water vapor and put into contact with a catalyst. The paraffins addressed within the scope of the invention are in the range from C2 to C20, preferably in the range from C2 to C5.
A large number of catalysts that are used to dehydrogenate paraffins are known. Such catalysts have a thermally stabilized, inorganic oxide as the substrate material, an active component (preferably a metal of the platinum group), and one or more promoters. Active Al2O3, which has an especially large specific surface area, is often used as the substrate material.
U.S. Pat. No. 4,788,371, describes a catalyst and a method for dehydrogenating paraffins in a water vapor atmosphere. The substrate of the catalyst comprises Al2O3 and is coated both with a noble metal (preferably platinum) and several promoters, which are selected from Group III or IV of the Periodic table and the gallium or germanium subgroup (preferably tin) and alkaline metals (preferably potassium or cesium). The dehydrogenation method described in this reference can function in the presence of a limited amount of oxygen, which is used to heat the reaction zone by combusting hydrogen.
From U.S. Pat. No. 5,220,091, a catalyst and a method for dehydrogenating C2 to C8 paraffins in the presence of water vapor is known. The catalyst used here comprises platinum (approximately 0.7 weight %) as well as zinc aluminate and potassium aluminate. In the dehydrogenation of isobutane (iC4), a conversion rate of 50% and a selectivity of 94 mol-% was attained; the pressure was adjusted to P=3.5 bar, the temperature was adjusted to T=571xc2x0 C., and the ratio of steam to isobutane (mol) was adjusted to 3.96. After a cycle time of 7 hours, the catalyst had to be subjected to a reactivation treatment by oxidative regeneration.
A further method and a catalyst for dehydrogenation of organic compounds is described in European Patent Disclosure EP 0 568 303 A2. This method uses a hydrogen atmosphere. The catalyst contains nickel and various promoters of Groups I-VIII of the Periodic table on a non-acid substrate material (base-treated Al2O3, zeolites, etc.). The special feature of the technology described in this reference is many dehydrogenation zones with intermediate zones for oxidizing hydrogen produced, on a special catalyst. The best results in the dehydrogenation of isobutane were obtained with a nickel catalyst (3.4% Ni and 3.4% Cr on a Ba-exchanged zeolite L), using a temperature of T=602xc2x0 C., a molar ratio H2/iC4=6 and a space velocity of WHSV=650 hxe2x88x921. Over an operating duration of 6 hours, the conversion rate was 30-36.6% and the selectivity was 75.1-83.4%. For an operating duration of 50-65 hours, the conversion rate was in the range from 22.2-27.9% and the selectivity was in the range from 78.8-81.1%.
Another catalyst and a method for dehydrogenation of hydrocarbons is known from International Patent Disclosure WO 94/29021. The method operates in a water vapor and hydrogen atmosphere, using a platinum catalyst, which as promoters contains elements of the tin subgroup and alkaline metals (potassium, cesium). The special feature of the catalyst is a special substrate material, which comprises a mixture of magnesium oxide and aluminum oxide. This composition requires a special pretreatment of the catalyst, which comprises a reduction with hydrogen, a calcination in an O2 atmosphere, and another reduction (called an ROR treatment). With this ROR treatment, the catalyst has an activity three times higher than without this treatment. The dehydrogenation of propane (C3) with the aid of the described catalyst, at a temperature of T=600xc2x0 C., a pressure of P=1 bar, a space velocity WHSV=1.3 hxe2x88x921 and a ratio of H2/H2O/C3=0.14/1.2/1 and an operating time of 25 hours, led to the following results: The propylene yield was 55.5 mol-%, and the selectivity was 96.1 mol-%. A comparative test described in this reference, using a catalyst known from U.S. Pat. No. 4,788,371, under otherwise identical conditions, led to a propylene yield of 25.7 to 29.7 mol-% and a selectivity of 95.0 to 95.9 mol-%. Thus WO 94/29021 represents the performance standard thus far in the field of catalytic conversion of paraffin hydrocarbons into corresponding olefins.
The object of the present invention is to disclose a catalyst for converting paraffin hydrocarbons into corresponding olefins that not only assures high effectiveness, or in other words has a good conversion rate and good selectivity, but furthermore exhibits high operating stability; that is, it can be used over comparatively long cycle times before having to be subjected to a reactivation treatment. The production of the catalyst should be as simple as possible. A method for converting paraffin hydrocarbons into corresponding olefins, which leads to good olefin yields and can be operated over cycle times that are as long as possible before catalyst reactivation has to be done is also to be disclosed.
In terms of the catalyst, this object is attained by the characteristics recited in claim 1, and in terms of the method, it is attained by the characteristics recited in claim 16. Advantageous features of the invention are defined by the dependent claims.
In the course of the tests that led to the present invention, it was discovered that catalysts known per se on Al2O3 substrates, which have platinum, a metal of the germanium or gallium group (preferably tin or indium) and an alkali metal (preferably potassium or cesium) can be improved substantially in terms of their activity by the addition of certain promoters. Along with progress in increasing the catalytic activity, it can be noted as a particular advantage of the invention that no special activation treatment, such as the ROR treatment, is necessary in the production of the catalyst. Furthermore, in the use of the catalyst, there is no need to add hydrogen to the feed material. On the contrary, the catalyst functions quite reliably in the presence of oxygen. The production of the catalyst can be done by known methods on conventional substrate materials.
The calcined catalyst of the invention comprises a thermally stabilized substrate material, onto which a catalytically active component is applied. The substrate material is preferably aluminum oxide, in particular in the form of "THgr"-Al2O3. The catalytically active component comprises the material groups a) through g), explained in further detail hereinafter, in which the quantities are given in weight % and are referred to the total weight of the
The material group a) includes the elements Pt and Ir, which represents the substance that is catalytically effective in the narrower sense, while the other material groups can be considered essentially as promoters, which promote the catalytic activity. The catalyst must have at least one of the elements of group a), specifically in a quantity of from 0.2 to 2%. The element Pt is especially preferred. It is recommended that the content of the element or elements of material group a) be limited to 0.3 to 0.6%.
As the promoter in the catalyst of the invention, at least one of each of the elements listed in material groups b) through g) described below must be represented. The material group b) comprises the elements Ge, Sn, Pb, Ga, In and Tl. The content of material group b) in the catalyst is in the range from 0.2 to 5%, and expediently in the range from 0.5 to 2.5%. The use of Sn is especially preferred.
The material group c) includes the elements Li, Na, K, Rb, Cs and Fr and has a quantitative proportion of from 0.1 to 5%, and preferably 0.5 to 1.5%. The elements K and Cs from this material group have proved to be especially effective.
The material group d) includes the elements Fe, Co, Ni and Pd. Its content is in the range from 0.2 to 5%, and preferably in the range from 1.0 to 3%. The use of Fe and/or Ni from this material group is especially expedient.
As a further promoter, the catalyst of the invention has a proportion (e) of P on an order of magnitude of from 1.0 to 5%. It is recommended that the P content be limited to from 2.0 to 4.0%. The material group f), whose quantity is limited to a proportion of from 0.2 to 5% and preferably to a range from 1.0 to 3%, includes the elements Be, Mg, Ca, Sr, Ba, Ra, and the group comprising the lanthanides. From this group, the elements Ca and Ba are preferred.
Finally, the catalyst has a proportion (g) of Cl on an order of magnitude of from 0.1 to 2%. The element Cl is a component that does not act as a promoter in the strict sense of the word, but that improves the initial dispersion of the noble metal in the catalyst. On the other hand, Cl promotes undesired secondary reactions at the onset of use of the catalyst. The initial content should therefore be clearly limited.
With the present invention, a method for converting paraffin hydrocarbons into corresponding olefins is also proposed in which a stream of the paraffin hydrocarbons is mixed with water vapor and put into contact with a catalyst that has the above-described composition at a temperature in the range from 500 to 650xc2x0 C. and at a pressure of at least 1.0 bar (absolute). Expediently, the addition of H2 to the stream of paraffin hydrocarbons and water vapor that until now was often usual is expediently dispensed with. It is recommended that the molar ratio of the water vapor to the paraffin hydrocarbons be adjusted within a range from 0.5:1 to a maximum of 10:1, and preferably within a range from 1:1 to 6:1. It has proved to be especially advantageous to use the catalyst of the invention with feed materials that contain hydrocarbons of the group comprising the C2 to C6 paraffins. To improve the conversion, it is advantageous to add O2 to the stream of paraffin hydrocarbons, because the oxygen reacts with the liberated hydrogen and thus shifts the equilibrium of the reaction. A molar ratio of the paraffin hydrocarbons to the O2 in the range from 1:0.2 to 1:1.5, and in particular in the range from 1:0.3 to 1:0.7 has proved especially expedient.
The invention will be described in further detail below in terms of exemplary embodiments.